Level measurement using guided wave radar is used in a vast number of different applications. Products being measured range from relatively safe products such as water or grain to highly dangerous materials like ammonia or petrochemicals. The principle of measurement is that of a “time of flight” measurement of a short, high frequency pulse. This pulse is generated and interpreted by an electronic module that is connected to a wave guide in a tank who's level is being measured through a process connection. The waveguide can be a length of metal rope, a rod or a coaxial transmission line composed of a rod inside of a hollow metal tube.
The impedance of the transmission line changes at the interface of the medium being measured causing a reflection. This reflection returns to the electronics module and its flight time is used to calculate the level of material in the tank. The process connection serves many functions. One function of the process connection is to electrically connect the electronics, residing outside the tank, to the waveguide inside the tank. The electrical characteristics of the process connection are critical to proper performance of the device. The process connection in effect forms a coaxial transmission line composed of a central conductor and an outside concentric conductor. The space in between is filled with dielectric material, who's dielectric constant and physical dimension define the line's impedance. Materials used here are typically plastics and ceramics.
Another function of the process connector is to transfer axial load from the probe in the tank to the tank roof/wall. Features designed into the shape of the central conductor, dielectric material, and outer conductor are used to support and transfer loads exerted onto the waveguide by the material in the tank. The final and most important function of the process connector is to seal out the process, which can be at a high temperature and pressure.
Some designs, referred to as “dual seal” designs, incorporate redundant seals with a vent path. If the primary seal fails, there is a secondary, lower pressure seal, which allows the process to vent. The venting prevents the electronics housing from flooding and thus the conduit employed to run the communication/power cable to the gauge. The sealing of the process connection is especially challenging at high temperature and pressure.